Process for preparing spherical energetic compounds

ABSTRACT

A method for producing substantially spherical energetic compounds such as ammonium dinitramide (ADN) which minimizes the time during which the ADN is melted involves providing solid ADN, feeding the ADN at a controlled continuous rate to a heating means, melting the ADN, the melted ADN being fed continuously to a non-solvent cooling fluid maintained at a temperature below the temperature of solidification of the ADN, the cooling fluid agitated in a manner which promotes the formation of droplets of controlled size which solidify in the cooling fluid to produce substantially spherical ammonium dinitramide in a particle size corresponding to the droplet size.

TECHNICAL FIELD

This invention relates to gas generating and/or propellant compounds,and more particularly to the preparation of energetic compounds such asammonium dinitramide in substantially spherical form.

BACKGROUND OF THE INVENTION

Ammonium dinitramide is a non-chlorine containing oxidizer useful inrocket propellant and gas generating devices such as air bag inflators.For example, in U.S. Pat. No. 5,324,075, ammonium dinitramide isdescribed as a preferred gas generator for air bags. In U.S. Pat. No.5,316,749, a process for producing stable ammonium dinitramide salts isdisclosed for use in a smokeless rocket propellant. While ammoniumdinitramide compounds have useful functional properties, they sufferfrom physical limitations which renders their use impractical.

For example, ammonium dinitramide ("ADN") crystallizes naturally in theform of needles or plates which are not readily amendable to subsequentprocessing. For use as a propellant or gas generator, it is necessary touse solid particulate ammonium dinitramide of controlled size to obtainparticulate ammonium dinitramide of controlled size to obtainpredictable results. Particles in the range of about 10 to 1,000 micronsare considered useful. However, efforts to control crystallization or tophysically process solid ADN to obtain a selected particle size havebeen unsuccessful, as solid ADN cannot be ground or subject to otherphysical processing due to its low stability.

One method for producing particulate ammonium dinitramide is to melt ADNin a non-solvent fluid, vigorously stirring to disburse the ADN, thenrapidly cooling the mixture before the melted ADN coagulates.

This process is not suitable because melted ADN can decompose violentlyif it remains melted for too long a period of time. Also, to minimizethe potential damage if decomposition were to occur, the total amount ofADN in the fluid must be minimized. While possibly adequate forproducing laboratory size quantities of ADN, such a method cannot beused to produce the large quantities of material necessary, i.e. for useas a propellant.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a substantiallyspherical solid energetic compound such as ADN of controlled size, witha limited melt processing time.

It is a further object to provide a continuous process for producingsubstantially spherical energetic compounds such as ADN in large scaleprocessing equipment, to produce large quantities of useful material.

These and other objects of the present invention are achieved by amethod of producing a substantially spherical energetic compoundcomprising:

providing a solid energetic compound;

feeding the solid energetic compound in a continuous controlled rate toa heating means;

melting the energetic compound in the heating means;

providing an agitated cooling fluid maintained at a temperature belowthe temperature of solidification of the energetic compound; and

adding the melted energetic compound in a rate comparable to thecontrolled feed rate, into the agitated cooling fluid such that theenergetic compound forms droplets after entering the fluid, whichsolidify in substantially spherical form.

The agitated cooling fluid is stirred in an amount sufficient todisperse the liquid energetic compound into droplets of the desiredsize. A stream of the cooling fluid is fed to a filtration device toremove the spherical energetic compound, with return of the fluid forcontinued use in the process.

Utilizing the inventive method, the amount of the energetic compound,such as ADN, which is melted at any one time is minimized, and theenergetic compound is in a melted state for a very short time. Thisminimizes the potential for decomposition. Further the energeticcompound is readily separated from the cooling fluid by filtration, andsubsequently washed, dried and stored for use. By controlling the fluidtemperature, rate of addition, and degree of agitation, it is possibleto obtain a fairly consistent particle size in the preferred range offrom about 10 to about 1,000 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the process of the present invention.

FIG. 2 is an alternative process according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention, while hereinafter described in relation to ammoniumdinitramide, is also applicable to other energetic compounds which arethose that are potentially unstable and difficult to process intosubstantially spherical form. For example, 1, 3, 3--Trinitroazetidine isan energetic compound which may be processed in accordance with thepresent invention, and the invention is not limited to ADN.

The terms "spherical" and "substantially spherical" are usedinterchangeably refer to the formation of grains or particles of theenergetic compound, as typically classified by percent of passagethrough a particular mesh screen. As is well known, such particles areclassified by the amount of material which passes through a screenhaving a known open area. A perfect sphere shape is not necessary butrather a substantially spherical particle size.

The energetic compound, such as ammonium dinitramide, may be obtainedthrough known processes. For example, U.S. Pat. No. 3,428,667 to Hamillet al. describes the reaction of an ionic nitronium salt with a primaryorganic nitramine to form N,N-dinitramine having the general formula R-N(NO₂)₂ !_(n) where n is 1-2 and r is a monovalent or divalent organicradical.

Referring to FIG. 1, a solid energetic compound 1 in powdered form, isplaced in a fed hopper 2 which is connected to a conveyor 3. Theconveyor may be of any suitable type such as a screw conveyor, beltconveyor, etc. for transporting solid materials at a controlled rate.Controllability is necessary to assure delivery of a selected quantityof the compound at a selected rate.

A heating device is located at a discharge end of the conveyor. Theheating device shown is a double pipe heat exchanger 4, having twoco-axial pipes, an inner pipe 5 through which the compound is processed,and an outer pipe 6, defining a space 7 between the pipes, to form ajacket, through which a heating fluid 8 is passed, heating the contentsof the inner pipe. The heat exchanger 4 is operated at a temperaturesufficient to melt the delivered quantity of the energetic compound. Theenergetic compound enters the pipe as a solid powder and then is meltedwithin the inner pipe. Once melted, the compound flows to an outlet end10 of the inner pipe. For example, ADN melts at a temperature of about100° C., and it is preferred to maintain the heating device at fromabout 100° C. to about 110° C. so as to avoid overheating of theenergetic compound material as it passes through the heating device.

The heating device is preferably placed at a downward angle towards theoutlet end to allow gravity flow of the melted energetic compound, fromthe entrance end to the exit end 10 of the heat exchanger. The exit endis positioned over an opening 12 above a cooling tank 13.

The cooling tank 13 contains a non-solvent fluid 14 subject to mixing byan agitator 15. The cooling fluid is maintained at a temperature belowthe melt temperature of the energetic compound so that near immediatesolidification occurs when the melted energetic compound enters thecooling fluid. Preferably, the cooling fluid is maintained at atemperature of about 20° C. when processing ADN.

After the energetic compound has entered the cooling fluid, and beforesolidification, it is dispersed into substantially spherical droplets ofthe desired size by the agitator, which shears and distributes thedroplets prior to solidification. The agitation also maintains theseparation of the droplets until solidification occurs.

A portion of the cooling fluid is then fed by a pump 18 to a filtrationdevice 19 where the solid substantially spherical energetic compound 20is captured. The cooling fluid is then fed by a pump 21 back to thecooling tank.

Preferably a second filtration device 22 is available so as to allowcontinuous operation in the production of the spherical particles, withthe second filtration device brought on line to allow washing, dryingand removal of the solid spherical energetic compound from the firstfiltration device 19.

Utilizing the present invention, a continuous process for producing anenergetic compound, particularly ADN is achieved. Further, the amount ofenergetic compound subject to melting is minimized, as is the timeduring which the compound is in liquid form.

Referring to FIG. 2, an alternative embodiment of the present inventionis shown.

In this method, the energetic compound 23 is placed in a carrier fluid24 contained in a tank 25. The carrier fluid is preferably identical tothe non-solvent cooling fluid. Thus, the energetic compound is mixedwith the carrier fluid to produce a slurry within which the solidenergetic compound is dispersed. Preferably, the liquid is continuouslystirred by an agitator 26 to maintain the dispersion of the energeticcompound within the liquid. Then, a portion of the slurry is fed to aheat exchanger 27, such as the double pipe heat exchanger describedabove. As previously, a heating fluid 28 heats the mixture in the innerpipe above the melting point of the energetic compound, causing thecompound to melt. Since the energetic compound is not soluble in thecarrier fluid, it separates into a second liquid phase 29. The two phasefluid can then be fed from an outlet end 29 in a controlled liquid rateto a stirred cooling tank 31 where the energetic compound phase isdispersed by the agitator into droplets 32 which solidify in a coolingfluid 33, again producing a substantially spherical particle product. Asdescribed above, a stream of cooling fluid containing the solidspherical particles can be fed to a filtration device for recovery ofthe solid energetic compound and return of the cooling fluid to theprocess.

Various non-solvent fluids can be used to contain the solid energeticcompound, such as mineral oil, fluorocarbon oil, silicone oil, etc. Anynon-solvent fluid of sufficiently high boiling point may be used,"non-solvent" meaning that the energetic compound will not react withthe non-solvent at or above its melting temperature.

Utilizing the present invention, energetic compounds, such as ADN, canbe obtained in sizes of from about 10 to about 1,000 microns with therange dependent on the cooling fluid temperature, rate of addition,agitation speed, drop location, etc. For example, if fed closer to theagitator, more physical disruption of the droplet size is likely,reducing the particle size. On the other hand, less agitation combinedwith a higher rate of addition, will allow larger droplets to form andsolidify without breakage. For use as a propellant, it is preferred thatADN be produced with a particle size below about 425 micron, preferablywith the majority of particles in the 10-200 micron range.

EXAMPLE

One kilogram of powdered ADN containing 0.1 to 2% of stabilizer(hexamine) was placed in a feed hopper. A Teflon coated screw feedconveyor was used to deliver 100 grams per hour of ADN to a double tubeheat exchanger. Hot water circulated through the heat exchanger at atemperature of about 100° C., allowing the ADN to melt. A 1.5 millimetercapillary end was fitted to the exit end of the heat exchanger, topromote droplet formation. The heat exchanger was placed at an angle ofabout 20° C. to promote gravity flow of the melted ADN to the exit end.A cooling tank containing mineral oil was positioned below the exit end,the tank having cooling coils for maintaining the temperature of thetank at about 20° C. A baffle was provided adjacent to the capillary endto maintain a constant stirred environment adjacent to the droplet entrypoint. The agitator was of variable speed to adjust the degree ofagitation in the baffle area, having an adjustment range of 2,000-4,000rpm. The agitator was set at a speed of about 3,400 rpm, the agitatorhaving a diameter of 13/4". Using the apparatus described, of the solidsproduced 65% was the diameter of choice, substantially spherical ADNhaving a particle size of less than 425 microns.

The experiment was repeated with a 21/2 diameter agitator, set at 2,300rpm, to generate the same tip speed for shearing the droplets and thesame amount of substantially spherical ADN, having a particle size ofless than 425 microns was obtained.

Using the present invention, a continuous process for producingspherical energetic compounds such as ammonium dinitramide is provided,allowing production of large quantities of spherical material with aminimum melt quantity maintained at the melting temperature for aminimum time. The parameters for adjusting particle size are easilycontrolled, i.e. by increasing agitation speed to adjust the processingfor producing different size products, with total recycle of the coolingfluid. Thus, material and equipment costs are minimized while safety isenhanced using the process of the invention.

What is claimed is:
 1. A method for producing a substantially sphericalenergetic compound comprising:providing a solid energetic compound;feeding the solid energetic compound in a rate controlled continuousstream to a heating means; melting the energetic compound in the heatingmeans; providing a non-solvent cooling liquid maintained at atemperature below the solidification temperature of the energeticcompound; adding the melted energetic compound at a rate whichcorresponds substantially to the solid feed rate into the coolingliquid; and, controllably agitating the cooling liquid to disperse themelted energetic material into droplets of controlled size; such thatthe droplets are solidified in the cooling liquid into substantiallyspherical particles, the particles size corresponding to the dropletsize.
 2. The method of claim 1 further comprising providing a carrierliquid and mixing the solid energetic compound with the carrier liquidbefore feeding to the heating means.
 3. The method of claim 1 furthercomprising separating the solidified particles from the cooling liquid.4. The method of claim 1 wherein the melted energetic compound is addedto the cooling liquid close to a source of agitation.
 5. The method ofclaim 1 wherein the energetic compound is ammonium dinitramide.
 6. Themethod of claim 5 wherein the heating means is maintained at atemperature of about 100°-1100° C.
 7. The method of claim 6 wherein thecooling liquid is maintained at a temperature less than 100° C.
 8. Themethod of claim 5 wherein the cooling liquid is maintained at atemperature of from about 10°-90° C.
 9. The method of claim 5 whereinthe temperature of the cooling liquid is maintained at about 20° C. 10.The method of claim 1 wherein the non-solvent cooling liquid is selectedfrom the group consisting of mineral oil, silicone oil and fluorocarbonoil.